Conventional drilling typically uses single wall jointed drill pipe with a drill bit attached at one end. Weighted drilling mud or fluid is pumped through a rotating drill pipe to drive the drill bit to drill a borehole. The drill cuttings and exhausted drilling mud and fluid are returned to the surface up the annulus between the drill pipe and the formation by using mud, fluids, gases or various combinations of each to create enough pressure to transport the cuttings out of the wellbore. Compressed air can also be used to drive a rotary drill bit or air hammer.
However, in order to transport the drill cuttings out of the wellbore, the hydrostatic head of the fluid column can often exceed the pressure of the formation being drilled. Therefore, the drilling mud or fluid can invade into the formation, causing significant damage to the formation, which ultimately results in loss of production. In addition, the drill cuttings themselves can cause damage to the formation as a result of the continued contact with the formation and the drill cuttings. Air drilling with a rotary drill bit or air hammer can also damage the formation by exceeding the formation pressure and by forcing the drill cuttings into the formation.
Underbalanced drilling technology has been developed to reduce the risk of formation damage due to the hydrostatic head of the fluid column, which uses a mud or fluid system that is not weighted. Hence, drill cutting can be removed without having the fluid column hydrostatic head exceed the formation being drilled resulting in less damage to the formation. Underbalanced drilling techniques typically use a commingled stream of liquid and gas such as nitrogen or carbon dioxide as the drilling fluid.
Nevertheless, even when using underbalanced drilling technology, there still is the possibility of damage to the formation. The drilling fluid and drill cuttings are still being returned to the surface via the annulus between the drill pipe and the formation. Hence, some damage to the formation may still occur due to the continued contact of the drilling cuttings and fluid with the formation. As well, underbalanced drilling is very expensive for wells with low or moderate production rates.
Formation damage is becoming a serious problem for exploration and production of unconventional petroleum resources. For example, conventional natural gas resources are buoyancy driven deposits with much higher formation pressures. Unconventional natural gas formations such as gas in low permeability or “tight” reservoirs, coal bed methane, and shale gases are not buoyancy driven accumulations and thus have much lower pressures. Therefore, such formations would damage much easier when using conventional oil and gas drilling technology.
The present invention reduces the amount of pressure which normally results when using air drilling, mud drilling, fluid drilling and underbalanced drilling by using a two string drilling system, thereby greatly reducing formation damage.